It is customary for shoe manufacturers to reinforce the toe end of the upper to obtain improved wear and retention of shape. It is accepted practice throughout the shoe industry to obtain such reinforcement by the application of a thermoplastic stiffening resin, sometimes referred to as a box-toe resin, to the toe portion of the upper. The thermoplastic resin is applied as a melt and upon cooling forms a stiffly resilient reinforcing coating on the upper.
For a thermoplastic resin to be an acceptable stiffener in this application, the resin must satisfy the following requirements: first of all, the resin should have some adhesive character; it should have a low melt viscosity, preferably less than 150 poise at 190.degree. C.; the resin should set rapidly to prevent "welding" of stacked assemblages of the manufactured articles; and the resin must be stiff to impart and retain the desired shape, but it must also have sufficient flexibility, even at low temperatures, to resist cracking upon impact and to "snap back" to its original shape. This latter property, or more correctly, balance of properties is sometimes referred to as "rigid flexibility" and is perhaps the most troublesome and difficult property to develop in a thermoplastic resin, particularly in polyamide resins.